In SiC (silicon carbide), a high-temperature type (α-type) having a hexagonal crystal structure and a low-temperature type (β-type) having a cubic crystal structure are known. SiC is characterized, in comparison with Si, by having a high thermal resistance, a broad band gap, and a high dielectric breakdown electric field strength. For that reason, a semiconductor including an SiC single crystal is expected as a candidate material of a next-generation power device substituting for an Si semiconductor. In particular, α-type SiC has a band gap broader than β-type SiC and hence the α-type SiC attracts attention as a semiconductor material of an ultralow power-loss power device.
α-type SiC has a {0001} plane (hereunder referred to also as “c-plane”) as the principal crystal plane and a {1-100} plane and a {11-20} plane (hereunder referred to also as “a-plane” collectively) perpendicular to the {0001} plane.
A c-plane growth method has heretofore been known as a method of obtaining an α-type SiC single crystal. The “c-plane growth method” cited here means a method of using as a seed crystal an SiC single crystal in which a c-plane or a plane having an offset angle to the c-plane in a prescribed range is exposed as a growth plane and growing an SiC single crystal over the growth plane by a sublimation reprecipitation method or the like.
The problem has, however, been that, in a single crystal obtained by the c-plane growth method, a large number of defects such as micro pipe defects (tubular voids about several μm to 100 μm in diameter) and threading screw dislocations (hereunder referred to also merely as “screw dislocations”) are generated in the direction parallel to the <0001> direction. Meanwhile, in a c-plane grown crystal, many basal plane dislocations exist in the c-plane and they are complexly intertwined with the screw dislocations in the c-axis direction (Non-patent Literature 1).
In particular, a basal plane dislocation curves largely in a {0001} plane by intertwinement between dislocations. In the case where a basal plane dislocation curves in this way, when a substrate (usually sliced so as to form an offset angle of 4° to 8° to a {0001} plane in order to form an epitaxial film) for manufacturing a device is taken from a single crystal, it sometimes happens that one basal plane dislocation may be exposed at plural sites on the surface of the substrate (refer to FIG. 15). As a result, the dislocation is succeeded from the plural sites when the epitaxial film is formed (Non-patent Literatures 2 and 3).
Further, in the case where a basal plane dislocation curves, the basal plane dislocation is oriented to various directions crystallographically. When a device is manufactured with such a single crystal and the device is operated, a stacking fault is formed by the decomposition of the basal plane dislocation into partial dislocations oriented to a crystallographically stable direction (<11-20> direction) during the operation (refer to FIG. 16) and the degradation of the device characteristics (a forward degradation phenomenon in a bipolar device) may sometimes be caused (Non-patent Literature 4).
A line must not be a straight line in order that the line intersects with a plane at plural sites. It would be better for a line to be rectilinear in order to reduce the number of the intersection sites. Consequently, it is geometrically obvious that it is better to reduce the number density and the total length of a basal plane dislocation and make it rectilinear in order to prevent the basal plane dislocation from being exposed at plural sites on a substrate surface (refer to FIG. 17). Further, in the case where a basal plane dislocation is oriented to a crystallographically stable direction, the basal plane dislocation hardly decomposes into partial dislocations and hence it is desirable to orient the basal plane dislocation to such a crystallographically stable direction (refer to FIG. 18). It is estimated that influence on the characteristics of a device may be reduced by so doing.
Meanwhile, as described in Patent Literature 1, it is possible to reduce a dislocation density in a crystal by using a method (RAF method) in which a c-plane growth is performed after a repeated a-plane growth. Further, in Non-patent Literature 5, it is described that a basal plane dislocation tends to be oriented by the RAF method. In the literature, however, a measure for judging the existence of orientation and linearity is not obvious. Further, a dislocation density is still high, intertwinement with a threading defect occurs frequently. Although the tendency of orientation is recognized partially in each of dislocations, the linearity is not strong and many curved parts exist. Furthermore, such a region is limited to a region of the order of submillimeters.